1. Field of the Invention
This invention relates to prosthetic joints and, in particular, to a prosthetic joint system for use when bone replacement is required.
2. Description of the Prior Art
Various prosthetic joints are known which include means for replacing bone which the patient has lost as a result of disease, trauma, or bio-mechanical degeneration caused by, for example, a previous joint implant.
FIG. 1 shows a two piece prosthesis 14 of this type which includes a stem 16 and a bone replacement portion 18 which can be a bone graft or a shaped part composed of a plastic material, such as ultra high molecular weight polyethylene (UHMWPE). See "Link Endo-Model Total Femur Replacement", Link America, Inc., East Hanover, N.J., 1989, and "The Leinbach Horizontal Platform Femoral Component", Allo Pro Corporation, St. Petersburg, Fla., 1982.
FIG. 2 shows another prior art construction of this type which comprises a one piece metal prosthesis 10 which includes a proximal portion 12 which serves as a replacement for bone. Examples of prostheses of this general type include those sold by Howmedica under the product designations "Universal 32 mm Proximal Femur Total Hip Components with Polished Loops", "Muller Type 32 mm Proximal Femur", "Harris Calcar Replacement 32 mm Total Hip System", and "Averett Modified Head Neck 32 mm Total Hip System", and those sold by DePuy under the product designation "1031-30/44 Femoral Upper Third".
Although these prostheses have benefited many patients, problems have remained with their use. In particular, with reference to the prostheses of the type shown in FIG. 2, custom prostheses are often needed and even when customized, these prostheses may not fit the patient particularly well, especially in connection with achieving a tight fit between the prosthesis and the patient's bone. These prostheses provide the surgeon with only one choice of replacement at the time of surgery which limits the ability of the surgeon to respond to specific needs or problems ascertained after the surgery has begun. Also, custom prostheses take a substantial amount of time to fabricate and are expensive.
With reference to the prostheses of the type shown in FIG. 1, these can provide the surgeon with some degree of choice at the time of surgery. Typically, a family of stems and a corresponding family of replacement portions 18 are provided to the surgeon. Also, in some cases, parts of the replacement portion can be cut away at the time of surgery to accommodate different amounts of bone loss. See the Allo Pro system, supra. However, these prostheses still suffer from problems in achieving an optimum interface between the stem 16 and the patient's bone. In particular, because of the mechanical configuration of the prosthesis, porous coating of the stem to achieve long term fixation through bone ingrowth is inadvisable since it may reduce the endurance strength or the stem.
Femoral prostheses which include sleeves for engagement with bone are also known in the art. See, for example, Noiles U.S. Pat. No. 4,846,839 and Noiles U.S. Pat. No. 4,790,852. Such prostheses have not included separate bone replacement elements of the type shown in FIG. 1 but in some cases have included an integral bone replacement portion of the type shown in FIG. 2.